Unfortunately there is a problem with the Remote Map file for the AKAI MPK Mini mk3 in Reason 12.2.9. Fortunately, though, there is an easy fix:
Download the AKAI MPK Mini mk3 Remote Map file, attached below in this article, and move it to:
Windows:
C:\ProgramData\Propellerhead Software\Remote\Maps\Akai\
Mac:
Macintosh HD/Library/Application Support/Propellerhead Software/Remote/Maps/Akai/
Note: If there is no Remote folder in the paths above you will have to create it manually, by adding the following folder and sub-folders in the Propellerhead Software folder:
Windows:
Remote\Maps\Akai\
Mac:
Remote/Maps/Akai/
Then, add your AKAI MPK Mini mk3 again on the MIDI tab in Reason's Preferences.